The invention relates to a motion detector circuit for detecting indications of change in portions of the images described by a raster-scanning composite video signal.
Raster scanning is a procedure in which the two mutually orthogonal spatial-frequency dimensions of each of a succession of images are conformally mapped into a linear time domain.
Motion detectors are used in a variety of television image processing applications, and the motion detectors of this invention are useful in many of these applications. Of particular interest to the inventors is the use of a motion detector in the video tape recording of television broadcast or similar television signals, in which video tape recording folded-luminance signals are employed. The specification and drawing of U.S. Pat. No. 5,113,262 issued May 12, 1992 to C. H. Strolle et alii and entitled "VIDEO SIGNAL RECORDING SYSTEM ENABLING LIMITED BANDWIDTH RECORDING AND PLAYBACK" are incorporated herein by reference. The inventors sought to improve the motion signal separation circuit 106 as shown in FIG. 2 of that application over the circuitry comprising the elements 216, 218, 220 and 222 shown in FIGS. 3 and 4 of that application.
Each of various motion detectors known in the art include a frame comb filter (or possibly a field comb filter), which comb filter is followed in cascade by a low-pass filter that is used to suppress chrominance signal components in the response of the comb filter. The comb filter is used for obtaining a derivative, primarily with respect to time, of a composite video signal that at least at times is likely to include chrominance signal components. The low-pass filter may, as shown in U.S. Pat. No. 5,113,262, be constructed by differentially combining the input and output signals of a high-pass filter. The low-pass filter, operating in the time domain, has the following effect in the two-dimensional spatial-frequency domain that is mapped into the time domain by raster scanning. Essentially all higher spatial frequencies in the direction parallel to line scan are eliminated.
The time domain response of the low-pass filter is often supplied to a post-processor. In a typical post-processor those portions of the response of the low-pass filter which have a magnitude larger than a prescribed threshold value are detected to generate indications that the corresponding portions of the composite video signal describe portions of the image likely to be moving. Such indications may be single-bit in nature, generating a response in the spatial-frequency domain that changes with time and exhibits step transitions in the two mutually orthogonal spatial-frequency dimensions. In such case the post-processor often includes circuitry for performing spatial filtering procedures that generate a response to the single-bit indications in which the abrupt step transitions are replaced by more gradual transitions. Weckenbrock describes representative such circuitry in his U.S. Pat. No. 4,868,650 issued Sep. 19, 1989 and entitled "CIRCUITRY FOR EXPANDING THE EFFECT OF A VIDEO CONTROL SIGNAL IN MULTIPLE DIMENSIONS". Ko describes other representative such circuitry in his U.S. Pat. No. 5,083,203 issued Jan. 21, 1992 and entitled "CONTROL SIGNAL SPREADER".
When video tape recording television broadcast or similar television signals using frequency-folding of luminance signals to gain improved luminance bandwidth, as U.S. Pat. No. 5,113,262 describes, it is desirable to process moving portions of the television images differently from their stationary portions using an adaptive spatio-temporal filter. A spatio-temporal filter operates in the dimension of time between fields and in the two dimensions of image space that are parallel with and transverse to scan lines, respectively. When there is no or almost no change in a picture element from one frame to the next, frame averaging is used to reduce the noise level in the picture without loss of detail information and frame differencing is used to separate chroma from luma during recording. These are temporal filtering measures. When there is substantial change in a picture element from one frame to the next, as occurs when there is local image motion, frame averaging is dispensed with to avoid blurring, and transversal spatial filtering is used to separate chroma from luma during recording. There is a loss of vertical resolution in the luminance that accompanies the transversal spatial filtering, but human vision tolerates reduced spatial resolution in moving portions of an image. A motion detector is used during video tape recording to control the switching between the two modes of operation.
A motion detector of the type that uses a low-pass filter to suppress chrominance signal components in the response of the comb filter unfortunately also suppresses changes with time in the finer details of the luminance signal, such that the movements of areas of the image that have uniform surface texture are not easily detected. The invention is directed to avoiding this short-coming. The failure to detect the movements of areas of the image that have uniform surface texture causes them to be processed as stationary portions of the image; and the frame averaging reduces the moving uniform surface texture as if it were just noise, causing an undesirable diminuition of moving detail.